To keep them from doing the same thing again? How many people would that apply to? You kill your wife in a moment of passion, and you think people are worried you will do it again? That's not punishment. Making sure they don't go it again is called rehabilitation. Punishment is for, well, punishing somebody for what they did. And a lot people feel that life is so precious, that it can never be given back, that the murderer deserves the same as the victim got. And rightfully so. For less serious crimes, the loss of freedom is enough of a punishment.